Production of a solar cell begins with a bare semiconductor wafer such as a silicon wafer. During production, a metal pattern or grid is applied to the wafer, typically by means of screen printing or a buried contact process. The purpose of the metal pattern or grid is to collect electric current generated in response to excitation of the semiconductor structure of the solar cell by means of an external source of illumination. The metal grid typically comprises multiple fingers that are electrically connected to one or more bus bars.
For reasons of efficiency, on the one hand, it is desirable to maximize the light collection area of a solar cell. This dictates that the fingers should be thin, narrow and widely spaced to reduce shading of the light collection area. Also for reasons of efficiency, on the other hand, it is desirable that the metal fingers transfer electric current with minimal electrical losses. This dictates that the metal fingers should be thick, wide and closely spaced to minimize resistive losses. A large component of solar cell design is thus to achieve a good compromise between these opposing requirements.
Photovoltaic solar cell manufacturing is generally characterized by significant rejection rates of devices that fail to meet required specifications for efficiency and conventional testing methods are generally unable to determine the reasons for solar cells that exhibit poor efficiency.
Regions of a good photovoltaic device are laterally connected in parallel via low series resistance. One specific mode of photovoltaic device failure is that regions in a photovoltaic device become electrically isolated from or poorly connected to other regions in the photovoltaic device. For example, metal fingers break during manufacturing of solar cells, particularly during screen printing of optimal designs that are characterized by very thin fingers. In that case electrical current generated in the immediate vicinity of broken fingers cannot be effectively collected, which results in a loss in efficiency of the solar cell. Another failure mode results from a high contact resistance or specific areas within a solar cell with enhanced contact resistance. Current flow from the bulk of the semiconductor to the metal contacts causes a voltage drop that is determined by the contact resistance. Locally enhanced contact resistances reduce the efficiency of solar cells. Various potential sources of such locally enhanced contact resistances exist in industrially manufactured solar cells.
A need thus exists for methods and systems that enable identification of poorly connected or electrically isolated regions in indirect bandgap semiconductor devices. A need also exists for methods and systems that enable identification of broken metal fingers, bus bars and connections between fingers and bus bars in solar cells, which are a common problem occurring in industrial solar cells.